Reverse osmosis membranes have been used for removing ions, organic matter, or similar from raw water in applications such as desalination of seawater, production of ultrapure water, treatment of industrial water, and recovering discharged water (for example, Non-Patent Literature 1). A reverse osmosis membrane tends to decrease in permeate flux due to growth of microorganisms on the surface of the membrane or adsorption of organic matter thereto. A reverse osmosis membrane also tends to increase in modular differential pressure as a result of being fouled by suspended matter. This necessitates cleaning the reverse osmosis membrane regularly to recover the permeate flux and the pressure difference between the raw water and concentrate sides of the membrane element (hereinafter referred to as element differential pressure).
A spiral-type membrane element is known as the reverse osmosis membrane element. A known spiral-type membrane element is formed by disposing a permeate spacer between two reverse osmosis membranes, bonding three sides of the membranes with adhesives to form an envelope-like membrane. An opening of the envelope-like membrane is attached to a permeate collecting tube and the envelope-like membrane is wound together with a mesh-like raw water spacer feed spacer around the permeate collecting tube in a spiral manner. Raw water is fed to one end of the spiral membrane element and flows along the raw water spacer, and is consequently discharged as concentrated water from the other end of the spiral membrane element. While flowing along the raw water spacer, the water permeates the reverse osmosis membranes, thus being converted into permeate water. The permeate water flows along the permeate spacer in the envelope-like membrane and further into the permeate collecting tube, and is taken out from the end of the permeate collecting tube.
The spiral-type membrane element is equipped with the raw water spacer between the envelope-like membranes wound around the permeate collection tube so that channels for raw water are formed.
Increasing the thickness of the raw water spacer in the spiral-type membrane element reduces clogging of the raw water channels with suspended matter, thereby avoiding increase in the differential pressure, decrease in permeate flow rate, and deterioration in quality of permeated water. A spiral-type reverse osmosis membrane element has been in market whose raw water spacer has a large thickness for preventing clogging caused by suspended matter.
When the raw water spacer has a large thickness, the membrane surface area per element becomes smaller, and the permeate flow rate per element is reduced. Commercially available spiral-type reverse osmosis membrane elements have a membrane surface area of 42 m2 (440 ft2) or less.
Increasing the thickness of the raw water spacer does not have the effect of preventing decrease in permeate flux caused by the adsorption of membrane contaminants.
Reducing the thickness of the raw water spacer in order to increase the membrane surface area per element has also been proposed (for example, Patent Literature 1). In general, it has been considered that the channels tend to be clogged by suspended matter when the thickness of the raw water spacer is reduced. For elements whose raw water spacer has a small thickness, it has been unknown what characteristics they have and how they should be operated.
According to a membrane bioreactor (MBR) process, organic waste water such as sewage is treated with activated sludge in a biological treatment tank and the resulting mixture containing the activated sludge is separated into solid and liquid using an immersion membrane separation device immersed in the biological treatment tank. The process yields treated water with stable quality, and allows a high-load treatment with an increased concentration of activated sludge. A method for treating organic raw water has also been proposed in which MBR-treated water (water permeated through the membrane of an immersion membrane separation device) is fed to a reverse osmosis membrane device directly, and is subjected to reverse osmosis membrane separation (for example, Non-Patent Literature 2).
MBR-treated water contains a large quantity of high-molecular-weight organic matter having a molecular weight of 10,000 or more, which fouls membrane. Accordingly, the permeate flux decreases and the differential pressure of the membrane increases greatly over time in a reverse osmosis membrane device for treating MBR-treated water.